APEX Atmospheric observing run carried out on December 6, 2020. The panels from top to bottom show: atmospheric parameters from the observing log (blue and green); uncorrected spectra from the observations CLASS file (black); corrected spectra (blue, see text) plus ATM model prediction for airmass = 1.0 (red) and line positions for N₂O and O₃; ATM model simulations (red) and data vs model residuals according to Eq. (3) (blue dots, smoothed to 1 GHz); and atmospheric opacity breakdown according to those mo del results (several colors in the bottom panel). Zooms on selected frequency ranges, highlighted in yellow, are displayed in Fig. 4.